So I picked this up for $20 on a FB sale group because it turns on but doesn't go anywhere. The motors were getting 12v when you hit the pedal so I removed the gear boxes (going to regrease) and put power straight the motors. Still nothing. I'm assuming now that it went through water or something and burnt them up.

The Titan 550's seem to be the "go to" direct replacement. But here's my next question. The car didn't come with a battery, so it's sort of a clean slate for me to go up to 18v if I want to. Will the stock wiring in these handle 18v alright? And if so, what motors/batteries should I get for a decent setup?

I have the same kid trax charger. I just installed hpi gt 550 at 18v. I used the bypass diagram in this thread by 12vwiz. Also Be sure to get batteries with similiar AH. Buy a 19t pinion gear. viewtopic.php?f=10&t=3399.

I know my reply isn't answering any of your questions, but my son has one of these and I don't have low speed or reverse. So I am in the process of upgrading a Jeep Enforcer for my other son.....i didn't know where to start with Police car. I am going to use my trial and error on the Jeep, do a little more research on the Charger and then dig into fixing it. If I come up with anything helpful for the police car, I'll be sure to post it.

On a side note.....we got the charger brand new for my son. He rode it non-stop and the stock battery got to the point where it wouldn't charge, so I rigged it to run on a 12v lawn mower battery. He could get the front wheels to come off the ground. I didn't rig any type of in line fuse....just ran different fittings on the stock wiring. This eventually led to a car that only runs in high gear. I think it did something to the motor where it won't run in low or reverse.